1. Field of the Invention
The present invention relates to a hybrid integrated circuit device in which a sound noise is prevented, which would otherwise be generated due to a pulse voltage applied to a ceramic capacitor for bootstrap when a switching transistor, used in a discharge maintaining circuit of a plasma display for large-sized flat televisions or the like, is turned ON and OFF by a driving pulse.
2. Description of the Related Art
The plasma display is used on large-sized televisions or the like.
As shown in FIG. 2, an image control circuit 13 of a plasma display sends control signals to an address driver and a scan driver, and maps a luminous unit (cell). Next, the image control circuit sends control signals, as shown in FIG. 3A, to discharge maintaining circuits 12A and 12B that constitute a hybrid integrated circuit, and makes the cell discharge at intervals of four microseconds. Mapping is performed in every gradation of brightness (subfield=one millisecond), an image of one shot is achieved by discharging, and the mapping and the discharging are repeated at 60 frames per second, thereby achieving moving pictures.
As shown in FIG. 4, in the discharge maintaining circuits 12A and 12B, a supply voltage E from a power supply circuit is applied to bootstrap capacitors C1, C3, and C4. Furthermore, pulse signals of 250 kHz are applied to a terminal HIN and a terminal LIN of a driver IC 17 from the image control circuit 13, so that pulse signals of 250 kHz are generated from a terminal HO and a terminal LO of the driver IC 17.
The high-level pulse signal generated from the terminal HO of the driver IC 17 is applied to bases of switching transistors Q1 and Q2 in a pre-driver 18, so that the switching transistor Q1 is turned ON and the transistor Q2 is turned OFF. Accordingly, the discharge voltage of the electric charge that is charged in the bootstrap capacitor C1 while the switching transistor Q1 is OFF, is superimposed on the supply voltage via the switching transistor Q1, and is applied to gates of IGBT1 and IGBT2 in a driver 19 via the switching transistor Q1, so that the transistors IGBT1 and the IGBT2 in the driver 19 are turned ON.
In this manner, the pulse signal of 250 kHz generated from the image control circuit 13 is driven with the cycles of 1 kHz and 60 Hz, and controlling the electric discharge of the plasma display is performed.
The above-described discharge maintaining circuits 12A and 12B are incorporated on an insulated metal substrate for improving a heat dissipation effect to form a hybrid integrated circuit, as is described in Japanese Patent No. 2951102 by the present applicant.
FIG. 6 shows this hybrid integrated circuit. A ceramic capacitor C1 and a switching transistor Q1 are incorporated on a conductive path 32 on a metal substrate 30 that is insulated by providing an insulating layer 31 thereon.
FIG. 7 is a view showing that electrodes 34, 34 of the ceramic capacitor CI are fixed on the conductive paths 32, 32 formed on the metal substrate 30 by solders 33, 33. When the ceramic capacitor C1 is fixed to the conductive path 32 by the solders 33, 33, a solder crack might be generated, which causes insulation failure. Thus, in order to prevent the insulation failure being occurred, the ceramic capacitor C1 is covered with a hard resin 35 such as an epoxy resin.
FIG. 8 is a cross-sectional view of the ceramic capacitor C1. The ceramic capacitor C1 includes ceramic layers 35, 35 laminated between the electrodes 34, 34. Ceramic is widely used for capacitors, because of a high dielectric constant and a high reliability. However, when a driving pulse of 1 kHz and a driving pulse of 60 Hz are applied to the electrodes 34, 34, the ceramic layers 35, 35 will be distorted because of the piezoelectric effect. On the other hand, when the driving pulse is not applied, the ceramic layers 35, 35 will recover their original shape. Because such operation is repeated throughout the driving pulse signal application, the ceramic capacitor vibrates and generates a sound noise.
Japanese Patent Application Publication No. 10-201250 discloses that vibration is suppressed by using a ceramic capacitor in a chopper type boosting circuit.
However, in the hybrid integrated circuit used for a discharge maintaining circuit of a plasma display or the like, as described above, the ceramic capacitor is molded with an epoxy resin to protect from a solder crack. Because the epoxy resin is hard, vibration of the ceramic capacitor is transferred as it is. Furthermore, the hybrid integrated circuit used for the discharge maintaining circuit of a plasma display is attached on the metal substrate with the insulated surface in order to improve a heat dissipation effect. However, when the hybrid integrated circuit is formed on the metal substrate, the vibration of the ceramic capacitor resonates to vibrate the metal substrate, so that a sound noise is generated.